Effect of visible light on benzo(a)pyrene binding to DNA of cultured human skin epithelial cells

The ubiquity of the photosensitive carcinogen benzo(a)pyrene (BP) and visible light in the environment suggests that their interaction might lead to photoproducts harmful to humans. To test the combined impact of these two agents on human epithelial cells, binding of BP to cellular DNA was assessed following treatment of cultures with BP and low-intensity (4.6 watts/sq m) intermittent (12 hr daily, 3 to 5 days) cool white fluorescent light. Light exposure reduced the formation of covalent BP adducts 20-fold (from 150 to 7 pmol BP per mg DNA) in cells treated with 1 microgram BP per ml and completely inhibited cytotoxicity; even with 10 microgram BP per ml, light exposure markedly inhibited cytotoxicity. However, at low BP dosage (0.1 microgram/ml), covalent adducts (2 pmol/mg DNA) to cellular DNA are produced and their formation is not influenced by light. These adducts persisted for at least 7 days following treatment; this observation suggests that chronic low-level exposure of human epithelium to BP may lead to an accumulation of DNA damage.